Tandem pumped fiber amplifier

ABSTRACT

In an example, a tandem pumped fiber amplifier may include a seed laser, one or more diode pumps, and a single or plural active core fiber. The single or plural active core fiber may include a first section to operate as an oscillator and a second different section to operate as a power amplifier. The one or more diode pumps may be optically coupled to the first section of the single or plural active core fiber, and the seed laser may be optically coupled to the single active core or an innermost core of the plural active core fiber.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/200,462, filed Nov. 26, 2018, which is a continuation-in-part of U.S.patent application Ser. No. 15/782,756, filed Oct. 12, 2017, entitled:TANDEM PUMPED FIBER AMPLIFIER, now U.S. Pat. No. 10,211,591, issued Feb.19, 2019, which claims the benefit of U.S. Provisional Application No.62/408,046, filed Oct. 13, 2016, each of which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present disclosure relates to fiber amplifiers.

BACKGROUND

High average power fiber lasers with diffraction-limited beams that aresuitable for High Energy Laser (HEL) applications are currentlyprimarily limited in output power by Stimulated Brillouin Scattering(SBS) and Modal Instability. Some known systems have been effective toscale power to greater than the 2 kW level by mitigating SBS. However,Model Instability may still bottleneck power-scaling in regularlarge-mode area (LMA) fiber, e.g., non-photonic-crystal fiber (PCF)and/or photonic bandgap (PBG) fiber. Specifically, Modal Instability maylimit diffraction limited output power to a threshold near 2 kW, such as˜2.2 kW for 20 μm core step-index dual-clad fiber laser.

BRIEF DRAWINGS DESCRIPTION

The accompanying drawings, wherein like reference numerals representlike elements, are incorporated in and constitute a part of thisspecification and, together with the description, explain the advantagesand principles of the presently disclosed technology.

FIG. 1 illustrates a tandem pumped fiber amplifier.

FIG. 2 illustrates a cross-section view of a coaxial dual active corefiber that may be utilized in a tandem pumped fiber amplifier, in someembodiments.

FIGS. 3A-C illustrate, respectively, a graph of calculated signal powerand tandem pump power inside a final power amplifier along with upperstate population for a tandem pump fiber amplifier, a block diagram ofthe tandem pump fiber amplifier, and a cross-section view of a dual-coreand all-glass-fiber of the tandem pump fiber amplifier.

FIG. 4 illustrates a refractive index of a fiber similar to the fiber ofFIG. 3C.

FIG. 5 illustrates a cross-section view of a single active core fiberthat may be utilized in the tandem pumped fiber amplifier of FIG. 1, insome embodiments.

FIGS. 6A-C illustrate, respectively, a graph of calculated signal powerand tandem pump power inside a final power amplifier along with upperstate population for a tandem pump fiber amplifier, a block diagram ofthe tandem pump fiber amplifier, and a cross-section view of a singleactive core and all-glass-fiber of the tandem pump fiber amplifier.

DETAILED DESCRIPTION

Some embodiments of a tandem pumped fiber amplifier may include a seedlaser, one or more diode pumps, and a single or plural active corefiber. The single or plural active core fiber may include a firstsection to operate as an oscillator and a second different section tooperate as a power amplifier. The one or more diode pumps may beoptically coupled to the first section of the single or plural activecore fiber, and the seed laser may be optically coupled to the singleactive core or an innermost core of the plural active core fiber.

Some embodiments use a single active core oscillator and single activecore tandem amplifier (core pumped). Core tandem-pumping may providerelatively high absorption and/or efficiency. As a result, fiber lengthmay be relatively short, which may provide a relatively large margin fordeleterious nonlinear effects such as SBS, SRS (stimulated RamanScattering), FWM (four wave mixing), SPM (self-phase modulation), or thelike, or combinations thereof. One embodiment provides a greater than 4kW single narrowband fiber amplifier.

Any tandem pumped fiber amplifier described herein may be less bulky(e.g., smaller anchor not as heavy) and/or less costly than a systemusing a 10 kW single mode fiber laser in a regular LMA fiber usingnumerous 1018 nm single mode tandem fiber lasers as high brightness pumpsources. The tandem pumped fiber amplifiers may have lower powerrequirements and/or lower thermal dissipation requirements than suchsystems as well.

Whereas a system using a 10 kW single mode fiber laser in a regular LMAfiber using numerous 1018 nm single mode tandem fiber lasers as highbrightness pump sources may require a reduction in quantum-defectheating from the usual ˜9% in the 976 nm pumped system down to ˜4% inthe tandem 1018nm pumped amplifiers to achieve 10 kW, embodimentsdisclosed herein may not be subject to the same requirement in order toachieve 10 kW or greater. A system employing a tandem pumped fiberamplifier may be compatible with regular LMA fiber technology butscalable from a few kilowatts to 10 kW or more by scaling up themultimode diode laser pump power. As a result, embodiments describedherein may simplify manufacturing in high energy laser applications. Ina system employing a tandem pumped fiber amplifier, a final amplifierstage may be greater than 1 kW to address Model instability. A tandempumped fiber amplifier may push the single channel output power togreater than known thresholds created by Modality Instability, such as 3kW, 5 kW, 10 kW, or more.

Several advantages of this approach have been identified. First, thequantum defect in the power amplifier may be only 1.5% at 1030 nm whenpumped at 1018 nm, in some embodiments. At a nominal wavelength of 1.064nm, this quantum defect is about 4%. Which is less than half compared topumping at 976 nm (which may be associated with a 8.4% quantum defect).Secondly, the signal injected into the power amplifier may besignificantly greater than 0.1 kW. Both factors may provide a higherthreshold condition for Modal Instability. This may provide greater than10 kW of spectral beam combining (SBC) and coherent beam combining (CBC)combinable power. The tandem pumped fiber amplifier may use regular LMAfiber technology without compromising the total efficiency of thesystem. All of this can be achieved by pumping with low SWAP (size,weight and power) and low-cost multimode diode pumps rather than using amultitude of expensive and bulky single mode fiber lasers.

As used in this application and in the claims, the singular forms “a,”“an,” and “the” include the plural forms unless the context clearlydictates otherwise. Additionally, the term “includes” means “comprises.”Further, the term “coupled” does not exclude the presence ofintermediate elements between the coupled items. The systems, apparatus,and methods described herein should not be construed as limiting in anyway. Instead, the present disclosure is directed toward all novel andnon-obvious features and aspects of the various disclosed embodiments,alone and in various combinations and sub-combinations with one another.

The disclosed systems, methods, and apparatus are not limited to anyspecific aspect or feature or combinations thereof, nor do the disclosedsystems, methods, and apparatus require that any one or more specificadvantages be present or problems be solved. Any theories of operationare to facilitate explanation, but the disclosed systems, methods, andapparatus are not limited to such theories of operation. Although theoperations of some of the disclosed methods are described in aparticular, sequential order for convenient presentation, it should beunderstood that this manner of description encompasses rearrangement,unless a particular ordering is required by specific language set forthbelow. For example, operations described sequentially may in some casesbe rearranged or performed concurrently. Moreover, for the sake ofsimplicity, the attached figures may not show the various ways in whichthe disclosed systems, methods, and apparatus can be used in conjunctionwith other systems, methods, and apparatus.

Additionally, the description sometimes uses terms like “produce” and“provide” to describe the disclosed methods. These terms are high-levelabstractions of the actual operations that are performed. The actualoperations that correspond to these terms will vary depending on theparticular implementation and are readily discernible by one of ordinaryskill in the art. In some examples, values, procedures, or apparatus'are referred to as “lowest”, “best”, “minimum,” or the like. It will beappreciated that such descriptions are intended to indicate that aselection among many used functional alternatives can be made, and suchselections need not be better, smaller, or otherwise preferable to otherselections.

Examples are described with reference to directions indicated as“above,” “below,” “upper,” “lower,” and the like. These terms are usedfor convenient description, but do not imply any particular spatialorientation.

FIG. 1 illustrates a tandem pumped fiber amplifier 100. The tandempumped fiber amplifier 100 may include a seed laser 105 with a selectedcenter wavelength that may be pseudo random bit sequence (PRBS)phase-modulated to achieve necessary bandwidth to suppress SBS and keepcoherence length long enough for SBC and CBC applications. The selectedcenter wavelength may be in the range of 1020-1080 nm for Yb-dopedfiber, in some embodiments. The seed laser 105 may include apreamplifier (not shown) to generate sufficient power, e.g., greaterthan 20 W of seed power for amplification in the subsequent sections.

Fiber of the seed laser 105 may be coupled to (e.g., spliced) with fiberof a tandem oscillator pump and booster amplifier (TOP-boosteramplifier) 110, which may be coupled to (e.g., spliced) with fiber of apower amplifier 115. The TOP-booster amplifier 110 may include a firstsection of a single or plural active core fiber (e.g., a dual activecore fiber), and at least one set of one or more diode pumps opticallycoupled, e.g., end-coupled, side-coupled, or the like, or combinationsthereof, to the diode pump set.

In a plural active core example, some the cores of the plural activecore fiber (e.g., both of the cores of a dual active core fiber laser)may be arranged along a same axis or a different axis (e.g., coaxialwith the second core symmetrically surrounding the first core or withthe second core asymmetrically surrounding the first core). FIG. 2illustrates a cross-section view of a coaxial-type dual active corefiber 200 that may be utilized in the tandem pumped fiber amplifier 100of FIG. 1, in some embodiments. FIG. 5 illustrates a cross-section viewof a single active core fiber 500 that may be utilized in the tandempumped fiber amplifier 100 of FIG. 1, in some embodiments.

Referring now to FIG. 2, the coaxial-type dual active core fiber 200includes a first core 1 surrounded by a second core 201, In someembodiments, the first core 1 may have a first diameter, and the secondcore 201 may have a second different diameter (e.g., a second largerdiameter). In one embodiment, the first diameter may be ˜12.5 μm and thesecond diameter may be ˜35 μm.

The first core 1 may he doped differently than the second core 201. Inone embodiment, the first core 1 may include a first dopingconcentration and the second core 201 may include a second dopingconcentration associated with a higher absorption coefficient. In someembodiments, the first core 1 may include Yb-700 (Yb˜50×10²⁴ m⁻³) andthe second core 201 may include with Yb-1200 (Yb˜120×10²⁴ m⁻³).

In some examples, the first core 1 may have a numerical aperture (NA)that is not greater than an NA of the second core 201. In oneembodiment, the first core 1 may have an NA of about 0.05 or less andthe second core 201 may have an NA no less than 0.05, e.g., in the rangeof 0.05-0.10.

The fiber 200 may include a cladding 205 surrounding the cores 1 and201, and a jacket 210. The cladding 205 may be a glass-clad multimodepump guiding octagonal core with a third diameter that is greater thanthe second diameter of the second core, e.g., ˜800 μm.

Referring now to FIG. 5, the single active core fiber 500 may include afirst section (110, FIG. 1) and a second section (115, FIG. 1). Thefirst section 110 may include a core 501 having a diameter of ˜10-12₁μm. The core 501 of this section 110 may be doped with Yb-700(Yb˜50×10²⁴ m⁻³). The core 501 of this section 110 may have an NA ofabout 0.05 (in one embodiment, 0.07).

The fiber 500 in the first section 110 may include a cladding 505surrounding the core 501, and a jacket 510. The cladding 505 and thejacket 510 may be similar to any cladding or jacket described herein.The cladding 505 may be a glass-clad multimode pump guiding octagonalcore with a diameter that is greater than the diameter of the core 501,e.g., ˜800 μm.

The second section 115 may include a core 501 having a diameter of˜17-35 μm. The core 501 of this section 115 may be doped with Yb-700(Yb˜50×10²⁴ m⁻³) and/or with a doping profile of 80-85% confined doping.The core 501 of this section 115 may have an NA of about 0.05 (in oneembodiment, 0.07). The fiber 500 in the second section 115 may have asimilar cladding 505 and jacket 510 as in the first section 110.

Referring again to FIG. 1 the TOP booster-amplifier 110 may include thefirst section of the single or plural active core fiber, The firstsection may include a first fiber Bragg grating (FBG) and a second FBG(e.g., an HR (highly reflective) FBG and a PR (partially reflecting)FBG, respectively), which may include diameters corresponding to corestructure, e.g., the second core 201 (FIG. 2) or the core 501 (FIG. 5).The first and second FBGs may be centered at a selected wavelength toform a multi-mode oscillator which builds up necessary power for thetandem pump to be used by the power amplifier 115.

The selected wavelength may be less than a center wavelength of the seedlaser. A difference may be 3% or less (e.g., 2.3%) in some examplesand/or in a range of 0.1-6%. In this range, with a core structure havingsufficient dimensions to suppress Modal Instability, and SBS. Theselected wavelength may be in the range of 1010-1045 nm (e.g., 1018 nm),in one embodiment.

In some examples, the doping of the first core 1 (FIG. 2) or the core501 (FIG. 5) may be selected to generate only sufficient single modeseed power while it is bi-chromatically pumped by both the multimode 976nm pump as well as the 1018 nm tandem pump (which may be generatedwithin the core structure). The oscillator of the first section of thesingle or plural active core fiber may use the doped core structure toconvert most of the 976 nm multi-mode pump power into 1018 nm wavelengthwithin the core structure. Residual unabsorbed 976 nm pump (e.g.,several percent) may enter the cladding of the second section of thesingle or plural active core fiber (e.g., the power amplifier 115) andbe utilized, which may optimize overall efficiency (e.g., overallelectrical-to-optical power conversion efficiency). An o-o efficiencycorresponding to the first section may be 3% more efficient due to lowerquantum defect when generating 1018 nm wavelength compared to 1064 nmamplifiers.

FIGS. 3A-C (dual active core embodiment) illustrate, respectively, agraph 300 (FIG. 3A) of calculated signal power and tandem pump powerinside a final power amplifier along with upper state population for atandem pump fiber amplifier 350 (FIG, 3B), a block diagram of the tandempump fiber amplifier 350, and a cross-section view of a dual-core andall-glass-fiber 375 (FIG. 3C) of the tandem pump fiber amplifier 350.FIGS. 6A-C (single active core embodiment) illustrate, respectively, agraph 600 (FIG. 6A) of calculated signal power and tandem pump powerinside a final power amplifier along with upper state population for atandem pump fiber amplifier 650 (FIG. 6B), a block diagram of the tandempump fiber amplifier 650, and a cross-section view of a single activecore and all-glass-fiber 675 (FIG. 6C) of the tandem pump fiberamplifier 650.

Modeling of the tandem pump fiber amplifier 350 (FIG. 3B) has shown anabsorption of greater than 0.51 dB (of 976 nm and 101.8 nm) and greaterthan 15 dB (of 976 nm) in the second core. When the longest signalwavelength of 1080 nm at 20 W is launched into a. first section of thefiber 375, greater than 1.27 kW of signal may be generated which can beamplified in the second section of the fiber 375. Modeling of the pluralactive core embodiment of FIGS. 3A-C predicts greater than 83%conversion efficiency for the 976 nm multimode pump into a 1018 nmtandem pump, and power levels of greater than 12.5 kW may be generatedin the second core of the fiber 375 given a 10-meter-long oscillator.Results of this simulation are shown in the graph 300.

Modeling of the tandem pump fiber amplifier 650 (FIG, 6B) has shownsimilar results. In a ˜8 meter “effective” fiber length, a core withYb-700 may achieve absorption of greater than 0.51 dB (of 976 nm and1018 nm) and greater than 15 dB (of 976 nm) in the core. However, whenthe longest signal wavelength of 1080 mn at 20 W is launched into afirst section of the fiber 375, greater than 10 kW of signal may begenerated which can be amplified in the second section of the fiber 675.Modeling of the single active core embodiment of FIGS. 6A-C predictsgreater than 83% conversion efficiency for the 976 nm multimode pumpinto a 1018 nm tandem pump, and power levels of greater than 12.5 kW maybe generated in the core of the fiber 675 given a 10-meter-longoscillator and Yb-1200 doping. Results of this simulation are shown inthe graph 600.

Referring now to FIGS. 3A-C, FIG. 3C shows a refractive index of thefirst core, the second core, the inner glass clad, as well as relativedoping concentration in the cores, in this embodiment, the values376-379 may be, respectively, 880 μm, 800 μm, 35 μm, and 12.5 μm, andthe core region 381 may include a first doping profile of Yb-700˜50×10²⁴m⁻³ and the core region 382 may include a second different profile ofYb-1200˜120×10²⁴ m⁻³. In other embodiments, the values 376-379 may be,respectively, 880 μm, 800 μm, 35 μm, and 14 μm. FIG. 6C shows arefractive index of the core, the inner glass clad, as well as dopingconcentration in the core. In this embodiment, the values 676, 677, and679 may be, respectively, 660-880 μm, 600-800 μm, and 10-12 μm(oscillator fiber)/17-35 μm (power amplifier fiber), and the core region681 may include a doping profile of Yb-700˜50×10²⁴ m⁻³(oscillator fiberand/or power amplifier fiber) or 80-85% confined doping in the core ofthe power amplifier fiber.

The seed laser of any tandem pump fiber amplifier described herein maybe, for SBC or CBC, a single-mode semiconductor laser such as adistributed feedback (DFB) laser or a non-planar ring oscillator (NPRO)and phase modulated to suppress SBS (for other applications anyappropriate single mode seed, e.g., any 20 W single mode seed, may beused). The seed laser may be 1064 nm.

A TOP boost amplifier of the tandem pump fiber amplifier 350 (FIG. 3B)may include a dual active core and a plural clad, e.g., dual or tripleclad. The inner core may be doped to generate sufficient power at seedwavelength need. to enter the power amplifier stage. The outer multimodecore may be doped to convert a portion (e.g., most) of the 976 nm diodepump into the 1018 nm tandem pump. The tandem oscillator pump may use HRand PR FBG's to generate a ˜10 nm bandwidth multi-mode oscillator usinga combination of the inner and outer cores, which may be doped at thesame level or differently. FIR and PR. FBG's may have the same corediameter as the outer multimode core of the TOP-booster gain amplifier.The seed wavelength that is injected into this stage may be amplified byboth the 976 nm multimode pump as well as the 1018 nm tandem pump thatis generated inside the oscillator cavity, e.g., with differentabsorption coefficient determined by the rare earth dopant absorptioncross section at these wavelengths and the core-to-clad area ratios. Thepower scaling of this laser may be done by adding more diode pumps asneeded to reach target power values, e.g., 3 kW, 5 kW, 10 kW, etc. Insome embodiments, the “effective multimode HR-FBG” may be one of thefollowing types:

-   1. FBG written in Ge-doped fiber with a core diameter equal to 379    (FIG. 3C) or 479 (FIG. 4).-   2. FBG written in Ge-doped fiber with a core diameter equal to 379    (FIG. 3C) or 479 (FIG. 4) and spliced to a passive chirally-coupled    core fiber of the same core size so as to strip off the higher order    modes so that reflectivity of the FBG is greater than 99%.-   3. FBG written in grated-index (GRIN) fiber with an “effective core    diameter” equal to 379 (FIG. 3C) or 479 (FIG. 4).-   4. FBG written in GRIN fiber with an “effective core diameter” equal    to 379 (FIG. 3C) or 479 (FIG. 4) and spliced to a passive    chirally-coupled fiber of the same core size so as to strip off the    higher order modes so that reflectivity of the FBG is greater than    99%.-   5. FBG written in a multimode fiber with rings of alternating    high-index and low index shells.-   6. The seed wavelength that is injected into this stage may be    amplified by both the 976 nm multimode pump as well as the 1018 nm    tandem pump that is generated inside the oscillator cavity, e.g.,    with different absorption coefficient determined by the rare earth    dopant absorption cross section at these wavelengths and the    core-to-clad area. ratios. The power scaling of this laser may be    done by adding more diode pumps as needed to reach target power    values, e.g., 3 kW, 5 kW, 10 kW, etc.

The final power amplifier of the tandem pump fiber amplifier 350 may useall of the 1018 nm tandem pump in the inner cladding and a portion(e.g., all) any residual 976 nm light not absorbed. in the oscillatorbut guided in the outer clad. to amplify seed wavelength andmode-field-adaptor may be used to match the single mode beam in theTOP-booster fiber and the final power amplifier. Some embodiments mayuse a cladding light stripper (CLS) to strip off the residual 976 nmpump before the output endcap.

Some embodiments include a plural active core fiber in a ˜6 meter“effective” fiber length including a Yb-doped core region, an undopedinner clad region, and an outer clad region (e.g., an outer glass-clad).In one embodiment, the dimensions of the doped core region, the undopedinner clad region, and the outer clad region may be 20-25 μm, 70 μm, and400-600 μm, respectively (pump guide may be 800 μm). The plural activecore fiber may include a final power amplifier including a mode-fieldadaptor to match the mode to a power amplifier section of the pluralactive core fiber. A 1018 nm tandem pump may be guided in the inner cladregion and may pump the signal in the innermost core of the pluralactive core fiber.

FIG. 4 illustrates a refractive index 400 of a fiber that is similar tothe fiber 375 of FIG. 3C and includes a dual active core triple fiberclad. In this example, the values 476-479 are, respectively, 660-880 μm,600-800 μm/0.022 NA or higher, ˜35 μm (Yb-1200 doped annulus/0.05 NA),−10 μm. (Yb-700 doped/0.065 NA).

Referring now to FIGS. 6A-C, a tandem oscillator pump and boosteramplifier (TOP-booster amplifier) may include a core (10 μm and 25 μm)and a plural clad (e.g., DCF or TCF). For a seed wavelength of 1030-1080nm, the pump wavelength may be 1018-1030 nm. The TOP-booster amplifiermay be coupled to (e.g., spliced) with fiber of a power amplifier. Thepower amplifier may include a core (20-35 μm) and a plural clad (e.g.,DCF or TCF). An outer clad may be 400-600 μm. In some embodiments, thepower amplifier comprises an end-core & side-clad-pumped tandem poweramplifier with a wavelength of 1030 to 1080 nm. The power scaling ofthis laser may be done by adding more diode pumps as needed to reachtarget power values, e.g., greater than 10 kW.

The final power amplifier of the tandem pump fiber amplifier 650 may useall of the 1018 nm tandem pump in the inner cladding and a portion(e.g., all) any residual 976 nm light not absorbed in the oscillator butguided in the outer clad to amplify seed wavelength andmode-field-adaptor may be used to match the single mode beam in theTOP-booster fiber and the final power amplifier. Some embodiments mayuse a cladding light stripper (CLS) to strip off the residual 976 nmpump before the output endcap.

In view of the many possible embodiments to which the principles of thedisclosed technology may be applied, it should be recognized that theillustrated embodiments are only preferred examples and should not betaken as limiting the scope of the disclosure. I claim as my inventionall that comes within the scope and spirit of the appended claims.

1. A tandem pumped fiber amplifier for high energy laser (HEL)applications, the tandem pumped fiber amplifier comprising: an opticalfiber including at least one core, wherein a seed laser is opticallycoupled to the at least one core; the optical fiber including a firstsection to operate as an oscillator and a second different section tooperate as a power amplifier; and one or more diode pumps opticallycoupled to the first section of the optical fiber, wherein the at leastone core of the first section is doped to convert the one or more diodepumps into a tandem pump, wherein the one or more diode pumps and thetandem pump bi-chromatically pump the power amplifier; wherein aselected wavelength associated with the oscillator is less than a centerwavelength of the seed laser, wherein the at least one core isdimensioned to suppress modal instability at 2 kW or greater outputpower when a difference between the selected wavelength and the centerwavelength is in a range of 0.1-8% and a single active core is employed.2. The tandem pumped fiber amplifier of claim 1, wherein a centerwavelength of the seed laser is in a range of 1020-1080 nm.
 3. Thetandem pumped fiber amplifier of claim 1, wherein the selectedwavelength is in the range of 1010-1045 nm.
 4. The tandem pumped fiberamplifier of claim 1, wherein the oscillator comprises a single mode ormulti mode oscillator.
 5. The tandem pumped fiber amplifier of claim 1,wherein the one or more diode pumps comprise a set of diode pumps, andthe tandem pumped fiber amplifier further comprises: a combineroptically coupled between the set of diode pumps and the first sectionof the optical fiber.
 6. The tandem pumped fiber amplifier of claim 1,wherein the seed laser comprises a preamplifier to generate greater than20 w of seed power.
 7. The tandem pumped fiber amplifier of claim 1,wherein the seed laser includes a phase modulator and employs pseudorandom bit sequence (PRBS) phase-modulation.
 8. The tandem pumped fiberamplifier of claim 1, wherein the first section includes a highlyreflective fiber Bragg grating (HR-FBG) and partially reflecting fiberBragg grating (PR-FBG) associated with the oscillator.
 9. The tandempumped fiber amplifier of claim 1, wherein a final amplifier stage ofthe single active core includes a mode-field-adaptor to match the finalamplifier stage to a mode corresponding to the oscillator.
 10. A tandempumped fiber amplifier for high energy laser (HEL) applications, thetandem pumped fiber amplifier comprising: a seed laser optically coupledto a core of an optical fiber; the optical fiber including a firstsection to operate as an oscillator and a second different section tooperate as a power amplifier; and one or more diode pumps opticallycoupled to the first section of the optical fiber, Wherein the core isdoped to convert the one or more diode pumps into a tandem pump, whereinthe one or more diode pumps and the tandem pump bi-chromatically pumpthe power amplifier; wherein a selected wavelength associated with theoscillator is less than a center wavelength of the seed laser, whereinthe core is dimensioned to suppress modal instability al 2 kW or greateroutput power when a difference between the selected wavelength and thecenter wavelength is in a range of 0.1-8% and a single active core isemployed.
 11. The tandem pumped fiber amplifier of claim 10, wherein thefirst section includes a highly reflective fiber Bragg grating (HR-FBG)and partially reflecting fiber Bragg grating (PR-FBG) associated withthe oscillator.
 12. The tandem pumped fiber amplifier of claim 10,wherein a final amplifier stage of the single active core includes amode-field-adaptor to match the final amplifier stage to a modecorresponding to the oscillator.
 13. The tandem pumped fiber amplifierof claim 10, wherein a center wavelength of the seed laser is in a rangeof 1020-1080 nm.
 14. The tandem pumped fiber amplifier of claim 10,wherein the selected wavelength is in the range of 1010-1045 nm.
 15. Thetandem pumped fiber amplifier of claim 10, wherein the oscillatorcomprises a single mode or. multi mode oscillator.
 16. The tandem pumpedfiber amplifier of claim 10, wherein the one or more diode pumpscomprise a set of diode pumps, and the tandem pumped fiber amplifierfurther comprises: a combiner optically coupled between the set of diodepumps and the first section of the optical fiber.
 17. The tandem pumpedfiber amplifier of claim 10, wherein the seed laser comprises apreamplifier to generate greater than 20 w of seed power.
 18. The tandempumped fiber amplifier of claim 10, wherein the seed laser includes aphase modulator and employs pseudo random bit sequence (PRBS)phase-modulation.